This invention concerns a reed switch with a permanent magnet movable within a housing.
Reed switches have switch contacts enclosed in a glass tube. The contacts are moved away from or toward each other under the influence of a magnetic field. The magnetic field is usually generated by a permanent magnet which is moved with respect to the contacts by means of a movable carrier sleeve.
Thus, with a known type of reed switch (see "Maschine und Werkzeug", Vol. 23, 1983), the magnet carrier is movable in a housing and it extends out of one end of the housing so that it can be moved relative to the glass tube by means of a switch fork or a switch rod, etc. The inner end of the magnet carrier sleeve encloses a permanent magnet which moves with respect to the contacts in order to connect or separate the contacts. There is a close fit between the sleeve and the housing to assure that no dirt particles or liquid can enter the gap between the housing and the sleeve.
Such a known reed switch can be used to control a starter circuit of a combustion engine for a vehicle with a transmission which can be operated only when the gear is in neutral. In this case, the reed switch is set up in such a way that the sleeve is moved to a position opposite the contacts by means of a switching mechanism which is in neutral position and thus, closes the starter circuit which had previously been interrupted. As a result of a wide variety of transmission designs, the reed switch must, in some cases, be installed at a location where it is constantly in contact with lubricant oil containing iron abrasion particles.
In such a case, it is possible for the sleeve to pump contaminated lubricant oil into the interior of the reed switch where it can be trapped, as is the case when the lubricant oil is exposed to low temperatures and is thus highly viscous. Furthermore, iron particles may collect on the lateral surface of the sleeve, owing to the magnetic field, and completely block any sliding motion.